In this research we propose to investigate the properties of infectious viral-antibody complexes formed by some enveloped RNA viruses and to study the interactions between these complexes and cells of the murine immune system. The virus to be studies initially is the mouse lactic dehydrogenase virus (LDV), an enveloped RNA virus of the Toga B class which appears to replicate only in mouse macrophages and produces a persistent viremia in the animal. The specific projects designed to characterize the LDV-antibody complex include (1) an examination of the type of mouse antibodies and the nature of the viral antigens in the complex, (2) the role of the complex in maintaining the infection and possibly extending the host range of the virus in mouse cells, and (3) the mechanism of formation of the complex and the effect of immunization to LDV on the appearance of the complex. Investigations will be extended to other enveloped RNA viruses such as those associated with leukemias in mice. These studies should provide significant information concerning the mechanisms by which certain viruses are able to circumvent the host's immune defense system, and they may also provide a basis for further investigations of viral persistency and the diseases associated with slow viruses.